Angular dependence of surface acoustic wave characteristics in AlN thin films on a-plane sapphire substrates

AlN thin films have been grown on a-plane sapphire (Al2O3(112̄0)) substrates. X-ray diffraction measurements indicate the films are fully c-plane (0001) oriented with a full width at half maximum of the AlN(0002) rocking curves of 0.76–0.92◦. The epitaxial growth relationships have been determined by the reflection high energy electron diffraction analysis as AlN[11̄00]//Al2O3[0001] and AlN[112̄0]//Al2O3[11̄00]. Angular dependence of important surface acoustic wave (SAW) characteristics, such as the phase velocity and electromechanical coupling coefficient, has been investigated on the AlN(0001)/Al2O3(112̄0) structure. While the SAW is excited at all propagation angles with an angular dispersion of the phase velocity in the range of 5503–6045 m/s, a higher velocity shear-horizontal (SH) mode is observed only at 0◦, 105◦ and 180◦ off the reference Al2O3[11̄00] over a 180◦ angular period. The phase velocity of the SH mode shows dispersion (6089–6132 m/s) as a function of the SAW wavelength. Temperature coefficients of frequency are also demonstrated for both modes. PACS 81.15.Hi; 77.84.-s; 77.65.Dq